1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive, a method for preparing the pressure-sensitive adhesive, a polarizer, and a liquid crystal display.
2. Discussion of Related Art
A polarizer is an optical member included in a liquid crystal display (LCD). A polarizer has a multilayer structure that includes iodine compounds or dichromatic polarizing materials arranged in a predetermined direction; and protecting films (e.g. triacetyl cellulose (TAC)) for protecting a polarizing film or element. Further, the polarizer may include additional films, such as a phase difference plate, a wide viewing angle compensating plate, a brightness improving film and the like, to improve its performance.
Each film constituting a multilayer polarizer is made of materials having different molecular structures and compositions, and thus shows different physical properties. Particularly, these films lack dimensional stability under high temperature and/or high humidity conditions due to a difference in the shrinkage or expansion behavior thereof. As such, when a polarizer is fixed by a pressure-sensitive adhesive, the TAC layer, and any other layer, is subject to stress concentration at high temperature and/or high humidity. As a result, birefringence is induced, and light leakage takes place.
To solve these problems, techniques for providing the adhesive with a stress relaxation characteristic have been disclosed. Particularly, a method of designing the adhesive so as to cause high creep and easy deformation under external stress has been disclosed (e.g. Korean Patent Application Publication No. 1998-079266, Japanese Patent Application Publication No. 2002-047468, etc.).
However, this technique has a drawback in that cuttability or workability of the adhesive is significantly reduced. When the cuttability or workability of the adhesive is reduced, failure such as leakage or depression of the adhesive occurs when the polarizer is mass-produced, and thus the yield is remarkably lowered.
On the other hand, there have been attempts to design an adhesive with high hardness to minimize the light leakage.
For example, Japanese Patent Application Publication No. 2007-197659 discloses a technique in which an adhesive composition is prepared by adding a multi-functional acrylate, an isocyanate hardener, and a photo initiator to an acrylic copolymer containing a carboxyl group, and then an adhesive is prepared by UV curing the adhesive composition. Japanese Patent Application Publication No. 2007-212995 discloses a technique in which an adhesive composition is prepared by mixing a copolymer containing a hydroxyl group and a copolymer containing a carboxyl group at a predetermined ratio and adding a multi-functional acrylate, a multi-functional isocyanate hardener, and a photo initiator to the mixture, and then an adhesive is prepared by UV curing the adhesive composition.
However, the storage modulus (G′) of the adhesives disclosed in the prior art documents, is excessively increased, and thus initial adhesion is greatly reduced. Thus, durability is reduced at high temperature or humidity. Further, when the storage modulus of the adhesive is controlled to be low, the hardness of the adhesive becomes insufficient, and thus a large quantity of light leaks out.